1. Field of the Invention
The present invention relates to an optical pickup head commonly used for two or more kinds of optical recording media, the optical pickup head employing laser beams having different wavelengths according to the different optical recording media.
2. Description of Prior Art
In recent years, various kinds of optical recording media such as Laser Disks (LDs), Compact Disks (CDs) and Digital Video Disks (DVDs) have been undergoing continued development. Each kind of optical recording medium is standardized according to unique specifications, such as the thickness of the disk substrate. This means that an optical pickup head which records information onto optical recording media or reproduces information from optical recording media should be compatible with the different kinds of optical recording media available.
For example, consumers nowadays require a player capable of reading out recorded information from both CD disks and DVD disks. The following are important differences between the standards of the CD format and the DVD format.
(1) The difference in applied wavelengths: the wavelength for CD disks is about 780 nm, and the wavelength for DVD disks is about 650 nm.
(2) The difference in numerical apertures (NAs): the numerical aperture for CD disks is 0.37, and the numerical aperture for DVD disks is 0.60.
(3) The difference in thicknesses of the disk substrate from a recording surface (reflection surface) to an outer surface of the optical recording medium: the thickness of the substrate for CD disks is 1.2 mm, and the thickness of the substrate for DVD disks is 0.60 mm.
Consequently, the optical pickup head of a CD/DVD compatible player must be able to adapt to these differences in order to be able to read information contained in both CD disks and DVD disks.
Conventionally, there are two kinds of optical pickup heads that are used in CD/DVD compatible players. In the first kind of optical pickup head, an independent optical system is provided therein for each of the two kinds of optical recording media. That is, the optical pickup head has at least two objective lenses: one for a CD disk and the other for a DVD disk. The major drawback of this kind of optical pickup head is that it needs too many optical elements, which makes it unduly large and costly. In the second kind of optical pickup head, a single optical system contains shared components which enable reading of both CD and DVD disks. This kind of optical pickup head reduces the number of optical elements and simplifies the overall configuration. However, the optical performance of the optical pickup head is limited. For example, an objective lens of the optical pickup head has a single focal point with a fixed numerical aperture. The objective lens is suitable for one kind of optical disk, say a CD disk; but is unsuitable for the other kind of optical disk, say a DVD disk. For the DVD disk, the objective lens produces aberrations such as spherical aberrations.